Records of prehistoric earthquakes in sedimentary deposits in lakes
Historic records of earthquakes are too short to allow a true assessment of their recurrence intervals. Methods are needed, therefore, that will enable the seismicity of an area to be evaluated beyond the limit of historic records. One place where a record of ancient seismic activity might be preserved is in lake sediments. Strong earthquakes often cause a phenomenon in water-saturated sediments known as liquefaction. The seismic shaking tends to reorient the sediment grains into a more compact arrangement. This usually means that the sediment liquefies and is redeposited in a smaller volume; there is an accompanying explusion of water and, in cases of severe shaking, sediment-laden water. During a strong earthquake, deformation due to liquefaction may occur in the upper layers of lake sediment. If this deformation is preserved without further deformation as new layers of sediment are deposited in the lake and if one could later find and identify these layers, one would then have a record of prehistoric earthquakes in the lake beds.